Page 06 9. Which letter in the diagram represents the first stage in cell signalling for a peptide hormone molecule? C D B A extracellular intracellular 10. Red blood cells swell or shrink rapidly in response to changes in the water concentration around them. This is due to one type of water channel protein, Aquaporin 1 (AQP1). Each red blood cell can transport a total of 1 × 1014 water molecules per second and each AQP1 channel can transport 5 × 108 water molecules per second. How many AQP1 channels are present in the membrane of a red blood cell? A 2 × 105 B 5 × 105 C 2 × 106 D 5 × 106 11. Which of the following changes in the rate of the cell cycle could result in a degenerative disease? A A controlled increase B A controlled decrease C An uncontrolled increase D An uncontrolled decrease